Hi. This is my first post to Cycling forums. I think projecting your daily mileage loaded will not so much be a factor of what your previous weekend trips indicated, but rather a factor of the increase in 'quality' of your training rides leading up to your big tour...
In other words, what I experienced (at the age of 35) was that my weekend trip loaded mileage averaged about 40 to 50 miles (with a good amount of pain, I might add) at a time when my training rides (no load) were 20 miles or so. Contrast this with my training rides leading up to my TransAm ride, when I concentrated on rides of at least 2 hours and mileage averaging 40 to 50 miles. Upon embarking from Yorktown and kissing my wife and 3 yr old son goodbye, my main focus was not to crash with the 50 lbs of gear. Day 1, a half day, totaled about 25 miles when the next good camp ground past Jamestown seemed too far to attempt. Day 2 totaled about 80 miles - a nice ride with a little sightseeing of former president's houses and civil war battlefields included.
On Day 3, however, I surprised even myself, when I pushed forward for the campground south of Charlottesville VA for a mileage total of 101.5. Wow, I'd never planned it and really didn't expect to ride a century fully loaded. But it just happened. I think the fig newtons helped, as did the good quality training rides in the month leading up to departure. I took 2 advil and slept like a baby. Next day, I made the Cookie Lady's house right around lunch time...
My average daily mileage for the trip though totaled 45 to 50 miles. This was because, when I got to a place like Breckinridge, you tend to hang around for 4 days of loafing. The high mileage day was Hutchinson, KS, with a promise of lodging by a local church, I pushed it to 123 miles with a nice tailwind. I took the next day off, even though I didn't really need it.
The advice by "Nun" is all you really need...Enjoy the Ride.